x x i v I N T R O D U C T I O N T O V O L U M E 7

first systematic exposition of the theory, Einstein 1916e (Vol. 6, Doc. 30). Ein-

stein’s correspondence with Willem de Sitter played a central role in this develop-

ment, and in Einstein 1918c (Doc. 5) he defends his new views on the foundations

of the theory against a challenge posed by De Sitter. Responding to criticism on

another front, in Einstein 1918f (Doc. 9) he attempts to convince his skeptical col-

leagues of the suitability of his formulation of the law of energy-momentum con-

servation in general relativity.

What is striking about general relativity at this time is the extensive progress

made between 1916 and 1919 on a theory that introduced many novelties for phys-

ics, especially in its mathematical formulation. In 1917, Einstein brought cosmol-

ogy within the reach of the tools of theoretical physics (Einstein 1917b [Vol. 6,

Doc. 43]). In Einstein 1919a (Doc. 17) he introduces an extension of general rela-

tivity to explain both the large-scale structure and history of the universe, and the

structure of fundamental particles such as electrons. General relativity permitted

concrete theoretical descriptions of two entirely new physical phenomena, gravita-

tional waves and “frame-dragging,” also known as the Lense-Thirring effect

(Thirring 1918, Lense and Thirring 1918). An exact solution of the rather forbid-

ding Einstein field equations had been found for the gravitational field of a point

mass (Schwarzschild 1916), and at least two different approximation schemes had

been elaborated for performing real calculations within the theory, namely the lin-

earized (Einstein 1916g [Vol. 6, Doc. 32]) and the post-Newtonian (Droste 1916b)

approximations. Alongside these theoretical developments came serious efforts to

test two of the three celebrated predictions of general relativity, anticipated by Ein-

stein even before his discovery of the final version of the theory (the Mercury peri-

helion shift, the bending of light rays in a gravitational field, and the gravitational

redshift).

By 1918, Einstein found himself interacting with a large number of colleagues.

His papers thus contribute to a growing research community working on general

relativity, in contrast to his earlier isolation. By 1919, active groups had formed at

several European centers, including Leyden, Göttingen, Zurich, Rome, Vienna, and

Cambridge. Einstein followed the work of these colleagues closely, corresponding

with them (see Vol. 8) and occasionally responding to their papers in print (Einstein

1918b [Doc. 2], Einstein 1918d [Doc. 3], Einstein 1918c [Doc. 5], Einstein 1918g

[Doc. 8], Einstein 1918f [Doc. 9], and the last section of Einstein 1918a [Doc. 1]).

Einstein’s papers of 1918 address a range of topics, some of which were to

remain controversial for much of the rest of the century. Einstein 1918a (Doc. 1) is

particularly well known because it derives the famous quadrupole formula for the

flux of energy radiated by a source of gravitational waves. The calculational error

that had marred Einstein’s work on gravitational waves in 1916 (see Einstein 1916g